Growth kinetics of spherulitic apatite in some MgO-CaO-SiO2-P2O5 glasses
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Crystallization of glasses with compositions (wt%) of 11.2 MgO, 33.3 SiO2, (55.5−x) CaO, and xP2O5 (x=18.3, 16.65, 15.825 and 15.0) resulted in a spherulitic apatite phase with different crystal morphologies. An ellipsoidal morphology was observed for x=18.3, 16.65 and 15.825, and an anomalous morphology was observed for x=15.0. A metastable phase, which was similar in some characteristics to apatite, was also found for x=15.0. The growth kinetics of the spherulitic apatite crystals were investigated to explain the above observations. Both the dendrite arms along the  and [1 1¯20] directions of the apatite crystals showed constant growth rates in each glass. Growth-rate anisotropy was found between these two directions. The ellipsoidal shape of the apatite crystals is explained by this growth-rate anisotropy. The growth rates, and the growth-rate anisotropy, varied with the P2O5 content in such a manner that the changes in phase formation behaviour can be explained on the basis of the kinetic results.
KeywordsCrystallization SiO2 Apatite P2O5 Growth Kinetic
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